1. Field of the Invention
The present invention relates to a liquid crystal display and driving method thereof, and more particularly to a liquid crystal display and driving method thereof that is adapted to minimizing heat generation of a circuit and electromagnetic interface (EMI).
2. Discussion of the Related Art
A liquid crystal display device controls the light transmittance of liquid crystal cells in accordance with video signals, thereby displaying a picture. Among liquid crystal display devices, an active matrix type liquid crystal display device, which includes a switch device formed at each liquid crystal cell, is advantageous in displaying a motion picture because the switch device may be actively controlled. A thin film transistor (hereinafter, referred to as “TFT”) is mainly used as the switch device used in the active matrix type liquid crystal display device. Equations 1 and 2 show the disadvantages of liquid crystal display devices, such as slow response speeds due to characteristics of liquid crystals, such as viscosity, elasticity, and other properties.τr∝(γd2/(Δε|Va2−VF2|))  (Equation 1)
Herein, “τr” represents a rising time when a voltage is applied to a liquid crystal, “Va” represents an applied voltage, “VF” represents a Freederick transition voltage in which liquid crystal molecules start a tilt motion, “d” represents a cell gap of a liquid crystal cell, and “γ” (gamma) represents a rotational viscosity of liquid crystal molecules.τf∝(γd2/K)  (Equation 2)
Herein, “τf” represents a falling time when a liquid crystal is restored back to its original location due to an elasticity restoring force after the voltage applied to the liquid crystal is turned off, and “K” represents a unique elastic constant of liquid crystals.
TN (twisted nematic) mode is currently the most generally used liquid crystal mode in liquid crystal display devices. Response speed of TN mode liquid crystal may be changed by changing the properties of the liquid crystal material, a cell gap, and other operational parameters. Generally, however, rising time is about 20 ms to about 80 ms and falling time is about 20 ms to about 30 ms. Accordingly, the response speed of the liquid crystal is generally longer than a typical one frame period (NTSC: 16.67 ms) of an image. In other words, as shown in FIG. 1, even before the voltage charged in the liquid crystal cell reaches a desired voltage in one frame period, the image advances to the next frame, thereby creating a motion blurring phenomenon in which a screen in the motion picture becomes blurred.
As shown in FIG. 1, a liquid crystal display device of the related art cannot properly render a desired color and brightness because display brightness BL corresponding thereto does not reach the desired brightness due to the slow response speed of the liquid crystal when data VD is changed from one level to another level. As a result, in the liquid crystal display device, a motion blurring phenomenon in a motion picture is generated and the picture quality thereof decreases due to the reduction of contrast ratio.
In order to solve the slow response speed of the liquid crystal display device, an overdriving method, as shown in FIG. 2, modulates an input data VD to a preset modulated data MVD and applies the modulated data MVD to a liquid crystal cell to obtain a desired brightness MBL. The overdriving method increases |Va2−VF2| in Equation 1 above based on whether or not the data is changed, so that the desire brightness may be obtained corresponding to a brightness value of the input data within one frame period. Accordingly, the overdriving method compensates the slow response speed of the liquid crystal with the modulation of the data value to mitigate the motion blurring phenomenon in the motion picture.
FIG. 3 illustrates an overdriving circuit according to a related art. As shown in FIG. 3, the overdriving circuit includes a frame memory 33 for storing data from a data bus 32 and a lookup table for modulating the data. The frame memory 33 stores the data and supplies the stored data as a previous frame data Fn−1 to the lookup table 34. The lookup table 34 takes current frame data Fn and the previous frame data Fn−1 from the frame memory 33 as an address to select a preset modulated data MRGB, thereby modulating the data. The lookup table 34 includes a read only memory (ROM) and a memory address control circuit. Table 1 illustrates an example of the lookup table 34.
TABLE 1Classification01234567891011121314150023456791012131415151515101345678101213141515151520024567810121314151515153001356781011131415151515400134678911121314151515500123578911121314151515600123468910121314151515700123457910111314151515800123456810111214151515900123456791112131415151000123456781012131415151100123456789111314151512001234567891012141515130012334567810111315151400123345678911121415150001233456789111315
In TABLE 1 shown above, the leftmost column represents data of the previous frame Fn−1, and an uppermost row represents data of the current frame Fn. The overdriving circuit, as shown in FIG. 3, needs the frame memory 33 for storing the previous frame data. The frame memory 33 included in a liquid crystal display device is a major cause of increased circuit cost.
Further, the lookup table 34 may be embedded in a timing controller for controlling drive circuits of a liquid crystal display panel. In such a case, other problems exist, such as increased electromagnetic interference (EMI) in a data transmission path between the timing controller and the frame memory 33 and increased heat generation of the timing controller. In addition, size of the chip of the timing controller becomes large. This is because there is a large amount of data transition transmitted between the frame memory 33 and the lookup table 34 loaded within the timing controller.